Calculation of deposition rates in diode sputtering systems

Abstract

The scattering of sputtered atoms by the sputtering gas has been modelled to obtain values for the distances which the atoms travel normal to the sputtering target before their energies are reduced to the thermal energy of the gas. This distance increases with the mass and energy of the sputtered atom and with decreasing gas pressure; for a 5-eV atom of mass 80, it decreases from 42 cm at an argon pressure of 0.1 Pa to 0.44 cm at 10 Pa. For most diode sputtering configurations, the sputtered atoms are thermalized before reaching the substrate and the transport to the substrate is by diffusion. Relative deposition rates for substrates situated behind apertures or masks in these diffusive sputtering situations have been measured and compared with the solid angles subtended at the substrate by the effective aperture. The agreement between relative values is very good. Thus, the solid angle provides a simple measure for determining the effect of a mask on the deposition profiles. Significant changes in rates and profiles are possible. With a 5-cm-square aperture, the rate at the center decreases by 50% as the substrate moves 2 cm away from the aperture plane and there is a 30% variation in rate over the substrate. It is impossible to obtain a sharply defined edge using a mechanical mask.